The present invention relates to a method for adjusting a photodetector array, and a beam-splitting and detector structure for a line scan camera, which structure comprises a photodetector array which is protected by a casing from a. least the back, and adjusted to the focus plane of an objective and glued to the exit surface of the beam-splitting prism by means of a support and adjusting structure.
Based on the U.S. Pat. No. 4,323,918 and the Japanese Patent 63-90985, adjusting structures are known in which the photodetector array is glued to the exit surface of the beam-splitting prism by using a spacer between the photodetector array and the beam-splitting prism. In the U.S. Pat. No. 4,323,918, the spacer is placed between the beam-splitting prism and the photodetector array, and thus, the spacer must be accurately measured. In the solution according to the Japanese Patent 63-90985, the photodetector array is glued between holding blocks whereby it is possible, prior to glueing, to move the photodetector array with regard to the holding blocks, and thus it is unnecessary to use prism-specific spacers. The U.S. Pat. No. 4,916,529 also discloses a solution in which the surfaces of both the beam-splitting prism and the photodetector array have first been processed to be solderable, and following this, the adhering has been carried out with, for example, ordinary soldering tin which at the same time determines the distance of the photodetector array from the exit surface of the beam-splitting prism. The problem with the two solutions mentioned first is the need to use separate spacers, which both complicates the production and makes assembly more difficult. The problem with the solution mentioned latest, in turn, is the need to separately prepare both the beam-splitting prism and the photodetector array for the soldering, and especially the troubles of the actual soldering and the associated thermal problems.
The U.S. Pat. No. 5,315,384, in turn, discloses an adjusting method in which an alignment target generated by three point sources of light arranged in a line is employed, and on the basis of the image provided by the alignment target, a manual rough adjustment is first carried out for the photodetector array by means of a micrometer, and then a fine adjustment is carried out by an electromechanic manipulator. It is, however, difficult to obtain feedback from the photodetector array as the dark section between the source lights does not give any signal. Thus, it is necessary for the photodetector array to be located nearly on the same line with the source lights before a signal required for adjustment is obtained. Finally, the photodetector array is adhered to the exit surface of the beam-splitting prism by glueing between them a spacer through which ultraviolet light can penetrate. Said glueing is so carried out that the spacer is first glued onto the surface of the casing of the photodetector array with ultraviolet light-sensitive glue, and the opposite side of said spacer is also glued with ultraviolet light-sensitive glue to the exit surface of the beam-splitting prism. In this adjusting method, it is necessary to use spacers of carefully chosen material, of accurate measurements and manufacturing in order to avoid unnecessary reflections from the surfaces of the spacers and, on the other hand, in order to focus the photodetector array in an optimum manner. It is, furthermore, required that the spacers be chosen photodetector array specifically.
It is an object of the present invention to introduce a beam-splitting and detector arrangement, as well as an adjusting method for a photodetector array, by which method it is possible to get rid of spacers that are expensive and difficult to manufacture, and their photodetector specific selection, and nevertheless be able to carry out the adjusting reliably and accurately by a method which can easily be automatized and which does not require a manual coarse alignment phase. This object is achieved by a beam-splitting and detector structure according to the invention, which structure is characterized in that the support and adjusting structure of the photodetector array comprises a glue layer arranged between the edges of the casing surrounding the photodetector array and the exit surface of the beam-splitting prism. By means of this glue layer, it is on the one hand possible to adhere the photodetector array to the exit surface of the beam-splitting prism, and on the other hand, which is particularly essential as far as he present invention is concerned, when the glue is still fresh, the thickness of the glue layer can be regulated so as to adjust the photodetector array to the focus plane of the objective.
It is characteristic to the method of the invention for adjusting the photodetector array to the focus plane of the objective that it comprises the following steps, in which
a line-formed light, or a light of line-formed portions, is directed from the alignment target through the beam-splitting prism to its exit surface;
the photodetector array is positioned near the exit surface of the beam-splitting prism so that the element array intersects with the line-formed image of the alignment target on the exit surface;
the photodetector array is adjusted with regard to the line-formed image which is on the exit surface of the beam-splitting prism so that light levels of the detectors of the photodetector array essentially correspond to each other, and
the photodetector array is supported by its casing to the beam-splitting prism.
The method of the invention, thus, utilizes an electric signal produced by the photodetector array itself. Monitoring of said signal enables carrying out an optimal alignment. The method of the invention also enables compensation for Potential position errors between the photodetector array and its casing, because the photodetector array is now adjusted to the correct position (focus plane) regardless of its position in the casing. This is based on the fact that instead of solid spacers, the photodetector array is adhered by its casing to the exit surface of the beam-splitting prism by means of a glue layer most advantageously adjustable in thickness, which apart from enabling adjustments of distance also enables various kinds of inclinations of the casing with regard to the exit surface of the beam-splitting prism.
Thus, it is also possible that the method of the invention comprises an additional step in which the photodetector array is, after the adjusting to the focus plane, inclined to a desired angle with regard to the focus plane of the beam-splitting prism. By this method, the errors which are caused due to the line camera being positioned inclined with regard to the target, can now be compensated for by the desired inclination of the photodetector array.
What is significant in the method of the invention is that the solid spacer is replaced by a glue layer. The glueing advantageously comprises the steps in which
the photodetector array and the beam-splitting prism are drawn apart from each other
glue is spread on the edges of the photodetector array casing,
the photodetector array is moved back to the adjusted distance from the beam-splitting prism;
the glue is allowed to harden/the glue is hardened, and
the junction between the casing of the photodetector array and the beam-splitting prism is sealed with glue.
Accordingly, the adjusting is carried our prior to the glue spreading. This ascertains that the adjusting succeeds. Alternatively, and especially in cases where the adjusting has become a frequently repeated routine, the procedure may comprise the following steps in which
prior to positioning the photodetector array close to the exit surface of the beam-splitzing prism, the glue used in the gluing is spread on the edges of the photodetector array casing,
the glue is hardened after the adjusting, and
the junction between the casing of the photodetector array and the beam-splitting prism is sealed with glue.
The glue may be spread on the edges of the casing most advantageously as glue spots or as a glue line.
As the method of the invention is being employed, the method may additionally comprise a step in which prior to positioning the photodetector array close to the exit surface of the beam-splitting prism or prior to the possible spreading of the glue on the edges of the casing, the protective glass of the photodetector array casing is removed. Removing of the protective glass results in better sensitivity of the photodetector array, because one glass layer preventing the propagation of light, and reflections on the surfaces of said glass, are thereby removed.